1. Field of Invention
The present invention relates to a process for visualization of a blast wave which is the shock wave from an explosion.
2. Description of the Prior Art
The present invention relates to visualization of the shock wave propagating away from an explosion. The invention does not relate to subsonic or supersonic flows nor does it relate to shock waves surrounding supersonic bodies. Nevertheless, the common intent of visualization may create the impression that the techniques are anticipations of the present invention--they are not.
There is a wide assortment of techniques for visualizing air flows and shock wave passages. Flow visualization techniques include: a windsock at the airport; an injection of smoke into an airflow; a blowing of dye to stain a model body; a positioning of an array of ribbons in the airflow; an injection of traces of dye or small bubbles into a flowing liquid; and examples in shadowgraphy and schlieren photography (with and without laser illumination).
Flow visualization techniques are not prior art for this invention. However, gas flowing at supersonic speed does exhibit a shock wave pattern in angled duct, such as a jet engine inlet, in which it flows; also a shock pattern exists around a supersonic projectile that is moving in air or conversely. These techniques need to be mentioned because the shock visualization techniques in the supersonic flow problems are sometimes repeated in blast wave problems.
Schlieren photography has become a nearly indispensable tool for investigating the flow of gases. An aeronautical engineer uses the schlieren technique to find valuable information about shock waves accompanying projectiles. A combustion engineer uses the schlieren technique in studying how fuels burn and investigations of heat transfer are aided by the ability of schlieren photography to show the paths taken by air over a hot surface. In general, the schlieren technique can be used to advantage whenever it is desirable to visualize the flow of gases. Being optical, the schlieren process does not interfere with the subject being observed. Normal motion of gases is not impeded, as is the case when Pitot tubes or yaw heads are inserted in the gas stream to detect flow direction. Optical methods involving an interferometer and shadow photography are also commonly used for visualizing gas flow. The interferometer has the characteristic of producing an image in which the differences in density are proportional to the differences in refractive index in the field. Thus, it is adaptable to quantitative measurements. A major disadvantage of the interferometer for investigating gas flow is its high cost. Also much care must be taken in adjusting the instrument, and the results are usually difficult to interpret. Shadow photographs, on the other hand, are easily taken with a minimum of equipment, but the results are not very useful unless the subject has strong gradients in the index of refraction. Schlieren photography, intermediate between these two extremes, indicates the gradient in refractive index. Combination of the three methods sometimes are used; (SCHLIEREN PHOTOGRAPHY, Eastman Kodak Company Publication P-11, 1974, page 2).
Motion picture photography has utilized smoke trails from rockets to determine time histories of particle velocity behind the blast front. Displacement of these smoke trails can be seen clearly in FIG. 9--23. (ENGINEERING DESIGN HANDBOOK, "Explosions in Air, Part One" AMCP 706-181, chapter 9, pages 17-18). The shock wave pattern surrounding a projectile is favorably visualized with shadow photography. Examples from the U.S. Army Ballistic Research Laboratory's (BRL) wind tunnel (now demolished) are in Fluid Mechanics, Raymond C. Binder, Prentice-Hall, 4th edition, pages 242-245. The shadow technique uses a spark timed to illuminate a photographic plate when a projectile is between the spark and plate. The projectile casts an ordinary shadow; the shock waves and wake eddies leave a shadow caused by refractive effects. A more complicated setup uses a Mach-Zehnder interferometer (Fundamentals of Optics, Jenkins and White, McGraw-Hill, 4th edition, pages 283, 604.
A shock wave from an explosive burst on the ground propagates as a hemispherical, invisible disturbance. An elevated explosive will create a spherical wave. The shape of the explosive itself is irrelevant; the shock wave becomes hemispherical or spherical at a distance of several basic body dimensions, e.g., a few diameters or cylinder heights. At large distances from the charge, the shock wave, though grossly weakened by the geometrical dissipation, still causes refractive index changes in air. Those changes are still evident at least at the 10 psi pressure level. The ground range where that pressure occurs depends on the weight of the explosive. That range grows very slowly with charge weight, being proportional to the cube root of the weight. Hence, the 10 psi level with one pound of explosive occurs at a ground range that is found in standard tables (10 ft.); to double that ground range requires that eight pounds be exploded.
At the shock wave's instantaneous position, the refractive index of air changes and light is bent. A moving ripple can sometimes be photographed against the background of a clear sky. The shock wave's appearance is less noticeable than a ripple dropped into a pool of water, but not always. More often, a moving break in the background scenery is noticed. To improve the location spotting of the shock wave, a large backdrop of striped sails is filmed at high speed. In multi-ton explosions, if the sun is high in the sky, the camera will film a black line racing over pale ground. This phenomenon is an expression of the refractive change of air and the setup is a form of shadowgraphy.